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Why do metallographic examinations are required after high-strength bolts are tempered?

Apr 25, 2018

GB/T 3098.1-2010 "Mechanical Performance of Fasteners for Bolts, Screws, and Studs" emphasizes that the material requirements for Grade 8.8 and above products should have sufficient hardenability to ensure that the core of the threaded section of the bolt is hardened. About 90% of martensite is obtained before tempering. The degree of tempering after bolt quenching is often measured in terms of hardness in production. To achieve good hardenability, the uniformity of the quenched structure is particularly important for high strength bolts of grades 10.9 and above.

Some companies in the heat treatment quenching, did not achieve the maximum hardness of the steel martensite may be obtained, that is not to obtain the maximum hardenability (hardening capacity of steel quenching), just by adjusting the tempering temperature, lowering the tempering temperature to ensure the minimum bolt The tensile strength value is used to mask the poor heat treatment quality. For this reason, GB/T 3098.1 stipulates that it is necessary to obtain approximately 90% of the martensite structure before the bolts are tempered.

After quenching, the thickness of martensite can be evaluated according to JB/T9211-2008 "Medium and Medium Carbon Alloy Structural Steel Martensitic Grade". Due to the different austenitizing temperatures, the martensitic morphology and size are not the same.

Grade 1 belongs to the low austenitizing temperature. The quenched structure is occult pin martensite, fine pin martensite, and not more than 5% ferrite (volume fraction); while the grade 8 is superheated and thick. Lath martensite + coarse martensite. Normal quenching is controlled from grade 3 to grade 5. Its microstructure is fine lath martensite and lamellar martensite. Grade 6 has high impact toughness, yield strength and tensile strength, suitable for larger sizes and Highly hardenable bolts are required.

To assess the quality after quenching and tempering, GB/T13320-2007 “Metal Microstructure Rating Diagram and Evaluation Method for Steel Die Forgings” is generally adopted. After polishing, the sample is etched with a 2% to 5% nitric acid alcohol solution. The metallurgical organization is rated on a scale of 1 to 8, with the best in level 1 organization; the worst in level 8 organization.

The third group of rating charts of this standard is applicable to structural steel quenching and tempering parts, especially the inspection of high-strength bolts. When the assessed quality organization is between two levels, the next level is the decision level. For example, if the level is greater than Level 3 and less than Level 4, the level is determined as Level 4.

The metallurgical microstructure of the quenched and tempered material was observed under a light microscope at a magnification of 500 times. The qualification level can be agreed upon by both the supplier and the buyer, and there is no agreement that the grade 1 to level 4 is acceptable. Production practice shows that grades 1 to 3 are acceptance criteria for bolts that are serviced and have low temperature impact performance in low temperature environments. If there is a dispute during the rating, you can refer to the mechanical performance test results to determine.

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